1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly to a system that optically transmits a frequency-division-multiplexed signal obtained by frequency-division-multiplexing a plurality of signals.
2. Description of the Background Art
FIG. 11 is a block diagram exemplarily showing the configuration of a conventional optical transmission system for transmitting a frequency-division-multiplexed signal. As will be known from FIG. 11, this optical transmission system comprises a multiplexer 1100, an FM modulator 1101, an optical transmitter 1104, an optical receiver 1106, and an FM demodulator 1107. In the optical transmission system, an electrical transmission line 1102 connects the FM modulator 1101 and the optical transmitter 1104 to each other, and an optical transmission line 1105 connects the optical transmitter 1104 and the optical receiver 1106 to each other.
The operation of the conventional optical transmission system in the above configuration will be described below. The multiplexer 1100 frequency-division-multiplexes a plurality of signals, and outputs the resultant signal to the FM modulator 1101. The FM modulator 1101 converts the frequency-division-multiplexed signal into a frequency-modulated signal (hereinafter, referred to as “FM modulated signal”) having a predetermined frequency deviation through frequency modulation. After that, the FM modulator 1101 outputs the FM modulated signal to the electrical transmission line 1102. The optical transmitter 1104 receives the FM modulated signal through the electrical transmission line 1102, then converts the signal into an optical signal, and sends the optical signal to the optical transmission line 1105. The optical receiver 1106 receives the optical signal through the optical transmission line 1105, then converts the signal into an FM modulated signal which is an electrical signal, and outputs the FM modulated signal to the FM demodulator 1107. The FM demodulator 1107 demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal.
The optical transmission system in the above configuration is described in detail in “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems” by K. Kikushima et al. (IOOC'95 Technical Digest, Vol. 5 PD2-7, pp.33-34), and other documents. The optical transmission system converts a frequency-division-multiplexed signal into an FM modulated signal, and then optically transmits and demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal. The optical transmission system utilizes an FM gain in the FM transmission to improve the signal-to-noise power ratio (SNR) of the demodulated signal (i.e., the frequency-division-multiplexed signal), thereby enabling high-quality signal transmission.
Thus, the above-described optical transmission system can realize high-quality multi-channel signal transmission with an optical fiber.
However, the above-described system for optically transmitting an FM modulated signal has the following specific problems due to the properties of the FM modulated signal and the nonlinearity of an optical fiber.